This invention relates generally to optical isolators for use with lasers at power levels up to and in excess of 1 kW.
High power optical isolators have been long sought for use with industrial metal working lasers to protect them from deleterious reflections. Highly reflective molten metal pools produced by these lasers can couple reflected light back into laser sources. This coupling can be highly efficient even at non-normal incidence angles. These reflections can cause the laser to run unstable or, in extreme cases, irreparably damage the laser itself. This is especially true for recently available, highly efficient, high gain, kW class fiber lasers. Similar concerns can be expected for high brightness direct diode arrays that have been demonstrated to have sufficient power to initiate “key-hole” deep penetration welds in metals [R. K. Huang, B. Chann, J. D. Glenn “Ultra-high brightness, wavelength-stabilized, kW-class fiber coupled diode laser”, website technical paper at www.teradiode.com (January/2013), also available from TeraDiode Inc., 11A Beaver Brook Road, Littleton, Mass. 01460]. For these reasons optical isolators suitable for use with high power lasers are considered an enabling technology for many emerging industrial and defense applications.
As an example, due to their very high brightness and invariant beam parameters, fiber lasers have shown great promise to become the efficient high power laser sources required for viable Laser Additive Machining [“LAM”] systems. In LAM, a 3D metal part is built up by using a tightly focused high power laser to sinter fine metal particles together. The potential to make small quantities of prototype or replacement parts directly with a remotely located laser system out of a canister of metal powder and using a 3D computer model of the part has important implications for future manufacturing. The minimum feature size of the resulting metal part is directly related to how tightly the laser can be focused—which can be as small as tens of microns for high brightness fiber lasers. Further, it is beneficial to be able to continuously vary the power of the laser source over its complete range. Very high power is used to build up solid interior regions of the part quickly or to make large features and high tolerance surfaces rapidly, while lower power is used to make very fine features of small dimension. However, as noted above, the reflective nature of the materials used, coupled with the high sensitivity of fiber lasers to optical reflections, has made optical isolators essential to further development of these potential fiber laser LAM and other markets.
Typical optical isolators comprised a Faraday rotator to provide 45° of polarization rotation surrounded by polarizers that have their transmission axis rotated 45° relative to each other. Faraday rotation serves to rotate the polarization from a first polarizer onto the transmission axis of a second polarizer on the opposite end of the Faraday rotator in the transmission direction. Because Faraday rotation is non-reciprocal (the sense of rotation is the same for beams propagating through the Faraday rotator in either direction) backward propagating radiation transmitted through the second polarizer and the Faraday rotator towards the laser source is orthogonally polarized to the first polarizer and thereby rejected.
At wavelength λ and temperature T the angle of polarization rotation, θ, in a Faraday rotator is given by θ(λ,T)=V(λ,T)·L·H(T) where H(T) is the magnetic field in the direction of beam propagation, L is the length of the Faraday optic in this direction and V(λ,T) is the Verdet constant of the Faraday optic.
To date, optical isolators suitable for use with high power lasers have suffered from thermal effects within the isolator optical elements. Intrinsic absorption of laser radiation transmitted through bulk optical elements and coatings within an optical isolator causes a thermal gradient to occur across the beam spatial profile. Faraday rotator materials typically have a higher absorption coefficient than any other element within an optical isolator, hence the thermal gradient and resultant thermal effects are highest within the Faraday rotator. Because the Verdet constant V(λ,T) is a function of temperature, this thermal gradient will cause the polarization rotation θ(λ,T) to vary across the beam profile. This thermal profile is responsible for two additional deleterious thermal effects: thermal lensing and thermal birefringence.
Depending upon the specific requirements of a given application, thermal lensing in optical isolators can be of a concern with as little as 20 W of average optical power. Thermal lensing primarily results from a change in material refractive index with temperature [dn/dT] that is present within optical components. Stress birefringence and end effects also contribute to the thermal lens, however, since their contribution is small they are usually ignored.
The focal length of a thermal lens in a Faraday rotator scales as the square of beam radius. For this reason, past attempts to reduce the focal length of the thermal lens have used large beams in very large and expensive Faraday rotators. However, because thermal lensing does vary the beam parameters of a laser beam, it is most importantly, and practically, observed as a shift in the focal position of the optical system, ΔZTL, which focuses the laser beam. When expressed relative to the Rayleigh range zR of the focused spot, the relative thermal lens focal shift ΔZTL/ZR, has been stated to be independent of beam diameter for a given Faraday rotator material and linearly dependent upon average optical power [K. Nicklaus and T. Langer, “Faraday isolators for high average power fundamental mode radiation”, Proc. of SPIE Vol. 7578, (2010)]. For average power P in kW, and focused spot Rayleigh range ZR, the relative thermal lens focal shift for a diffraction limited beam is:ΔZTL/ZR=ALP  Eqn. 1
For a fixed wavelength. A in Eqn. 1 is a constant that has units of cm−1 kW−1 and is derived from measurable constants such as the thermo-optical coefficient (dn/dT), absorption coefficient and thermal conductivity. The length L of the optical material is expressed in cm.
The Rayleigh range ZR associated with a given waist radius ωo at wavelength λ is:
                              Z          R                =                              π            ⁢                                                  ⁢                          ω              o              2                                λ                                    Eqn        .                                  ⁢        2            
Assuming relative focal shifts ΔZTL/ZR that are small relative to the focal length of a final focusing lens, it is straightforward to approximate how the focused beam radius ω(ΔZTL) will increase at higher power, P, from a starting condition where the low power waist ωo is in the focal plane of the final focusing lens:
                              ω          ⁡                      (                          Δ              ⁢                                                          ⁢                              Z                TL                                      )                          =                                            ω              o                        ⁡                          [                              1                +                                                      (                                                                  Δ                        ⁢                                                                                                  ⁢                                                  Z                          TL                                                                                            Z                        R                                                              )                                    2                                            ]                                            1            ⁢                          /                        ⁢            2                                              Eqn        .                                  ⁢        3            
For a common high power Faraday rotator material such as terbium gallium garnet [“TGG”], the characteristic constant is typically found to be A=3.5 cm−1 kW−1 at 1060 nm when a TGG crystal is of high quality with absorption coefficient:α=0.0015 cm−1.For a beam of any diameter and power P (in kW) at 1060 nm, the relative focal shift is therefore ΔZTL/ZR=7P when using 2 cm of high quality TGG with a diffraction limited beam. For an application such as LAM where it is necessary to have a very tight focus at low power, but where it is also important to vary the laser power rapidly, the effects of thermal lensing can be readily seen to be very important. For example, for a diffraction limited 1/e2 collimated 1060 nm laser beam diameter of 10 mm into a final focusing lens of 500 mm focal length, the low power focused beam radius, ωo, according to diffractive theory will be 33.7 μm with a corresponding Rayleigh range zR=3.37 mm. In this case if the final focusing lens is positioned to focus the laser beam tightly at low power for sintering small, fine features, according to Eqns 2 & 3 the beam diameter at a power P=1 kW will be:
                                                                        ω                ⁡                                  (                                      Δ                    ⁢                                                                                  ⁢                                          Z                      TL                                                        )                                            =                            ⁢                              33.7                ⁢                                                                  ⁢                μ                ⁢                                                                  ⁢                                                      m                    ⁢                                                                                  [                                          1                      +                                                                        (                                                                                    7                              ⁢                                                                                                                          ⁢                                                              Z                                R                                                                                                                    Z                              R                                                                                )                                                2                                                              ]                                                        1                    ⁢                                          /                                        ⁢                    2                                                                                                                          =                            ⁢                              238                ⁢                                                                  ⁢                μ                ⁢                                                                  ⁢                m                                                                                    
Such large differences in focused beam radius can manifest themselves as visually noticeable changes in the feature size as well as measurable dimensional shifts in LAM fabricated parts.
Although a larger beam size due to such focal shift may be suitable for building up bulk interior sections of a part, it will be unable to sinter fine features and accurate surfaces quickly at high power. Hence, such large thermal lens focal shifts will generally require the power to be kept very low for all fine features and accurate surfaces, making the time to build a typical part impractically long. This is just one example of the numerous ways thermal focal shift due to thermal lensing in optical isolators can be unsuitable for industrial application. An optical isolator having low focal shifts due to thermal lensing at high power is therefore desired.
The other detrimental thermal effect occurring in high power isolators noted previously is thermal birefringence. The thermal gradient across the beam profile due to absorption leads to thermal strains in the isolator optical components at high power.
These thermal strains cause linear birefringence via the photoelastic effect. This thermal birefringence becomes the limiting factor determining the isolation ratio, and consequently the effectiveness, of an optical isolator at high power. Thermal birefringence scales with the square of each of the following incident parameters: incident power level P, Faraday optic length L and absorption coefficient α. Unlike thermal lensing, thermal birefringence is independent of beam size. For a TGG rod length of 1.6 cm and absorption coefficient of α=0.0015 cm−1, it has been shown that thermal birefringence will limit isolation to less than 25 dB with less than 200 W [K. Nicklaus and T. Langer, “Faraday isolators for high average power fundamental mode radiation”, Proc of SPIE, Vol. 7578, (2010)]. At increasing power levels isolation drops rapidly unless thermal birefringence is addressed in the optical isolator. A passive means of effectively compensating thermal birefringence with isolation greater than 25 dB for power levels well in excess of 1 kW has been described [E. A. Khazanov, “Compensation of thermally induced polarization distortions in Faraday isolators”, Quantum Electronics 29 (1) 59-64 (1999)]. In this method a 67.5° reciprocal quartz polarization rotator is preferentially used between a pair of identical 22.5° non-reciprocal Faraday rotators such that a linear polarization entering the first Faraday rotator is flipped by 90° upon entering the second Faraday rotator. This polarization flipping substantially cancels linear birefringence via the photoelastic effect from thermal strains in each Faraday rotator. A half-waveplate may replace the 67.5° quartz rotator to achieve the same effect, albeit with reduced birefringence compensation performance at high power. However, due to the strong wavelength dependence of the desired 22.5° Faraday rotation angle in the method above, this birefringence compensation method alone does not achieve high levels of isolation over broad wavelength ranges typical of the gain bandwidth of common high power fiber lasers (as much as 200 nm including Raman gain) or high power fiber coupled laser diode arrays using spectral beam combining.
A method for achieving high power broadband optical isolation is described in detail in U.S. patent application Ser. No. 13/673,755 filed Nov. 9, 2012, owned by the assignee of the present invention. The content of this patent application is incorporated herein by reference for all purposes. The method described therein uses an additional 112.5° quartz rotator of opposite rotation sense to that of the 67.5° quartz rotator at the center wavelength of operation λc. The 112.5° quartz rotator is located between either the input or the output broadband polarizer and the respective adjacent 22.5° Faraday rotator in the above thermal birefringence compensation scheme to simultaneously passively compensate the wavelength dependence of Faraday rotation and thermal birefringence at high power.
Alternative techniques to improve the high power capability of optical isolators exist and have been discussed. A first method for reducing thermal lensing and/or thermal birefringence is to use improved Faraday optic materials. Terbium aluminum garnet (“TAG”) in crystalline or transparent ceramic form offers potential advantages as a Faraday rotator material over TGG in the visible and near infrared spectral region. At 1 μm, the Verdet constant of TAG is 30% greater than TGG as well as optical absorption similar to TGG and improved thermo-optical properties [M. Geho, T. Sekijima and T. Fujii, “Growth of terbium aluminum garnet (Tb3Al5O12; TAG) single crystals by the hybrid laser floating zone machine”, J. Crystal Growth, V. 267, 188-193, (2004)]. Permutations of TAG, such as TSAG (where some scandium is substituted for terbium) and TSLuAG (where scandium and lutetium are substituted for terbium) in order to improve crystal growth and yield with only a minimal reduction in TAG Verdet constant have also been described in the literature [Encarnación G. Villora, “Faraday rotator properties of {Tb3} [SC1.95Lu0.05](Al3)O12, a highly transparent terbium-garnet for visible-infrared optical isolators” Applied Physics Letters 99, 01111 (2011)]. However, the Verdet constant and thermo-optic improvements that these materials offer relative to TGG are incremental. These materials may reduce thermal lens focal shifts below that of TGG by a factor of approximately two. Such a reduction in thermal lens focal shift is insufficient to make low thermal lens focal shift, kW class optical isolators of simple construction similar to presently available lower power TGG optical isolators. Further, new Faraday rotator materials such as TAG in crystalline or transparent ceramic form are difficult and expensive to bring to the market. To date, TGG remains presently the most proven, broadly available high power Faraday rotator material for the visible and near infrared spectral region.
For conventional circularly symmetric laser beams, optical isolators typically use rod shaped Faraday rotator optical elements. In a quest to reduce thermal gradients across a beam, other Faraday rotator optical element geometries have been proposed. The most studied of these have been slab geometries and segmented discs with liquid cooled optical faces [E. A. Khazanov, “Investigation of Faraday isolator and Faraday mirror designs for multi-kilowatt power lasers,” in Solid State Lasers XII, R. Scheps, ed., Proc. SPIE 4968, 115-126 (2003)]. In slab geometries a highly elliptical laser beam is transmitted through slab shaped Faraday rotator optical elements having a high aspect ratio (width:thickness≧15) rectangular aperture. By removing heat only from the two large non-optical surfaces of the slab while thermally insulating the other two small non-optical edges, a temperature gradient across the beam in the thin slab dimension only is produced. This thermal gradient along one axis is substantially reduced with high aspect ratio slabs compared to that of a conventional rod shaped Faraday rotator with circular beams. Although reduced in magnitude, a cylindrical thermal lens is formed which can be more difficult to focus properly in an optical system. Additionally, either expensive cylindrical lens systems that are subject to additional thermal lens considerations, or complicated off axis spherical mirror systems prone to aberration at the desired high aspect ratios are required to first convert conventional circular beams to a highly elliptical beam and then back to a circular beam as desired by conventional laser use. Slab geometry Faraday isolators offer some utility when used with slab geometry solid state laser systems that already employ elliptical beams or with linear arrays of circular beams. However, the factors noted above have kept slab geometries from being broadly employed with conventional circular beam high power laser sources.
Another method to reduce thermal gradients within Faraday rotators is that of segmented disc Faraday rotator optical elements where heat is removed through the optical faces by flowing cooling gasses over them. In theory, thermal gradients occur primarily in the direction of beam propagation [E. A. Khazanov, “Investigation of Faraday isolator and Faraday mirror designs for multi-kilowatt power lasers,” in Solid State Lasers XII, R. Scheps, ed., Proc. SPIE 4968, 115-126 (2003)]. This greatly reduces the thermal gradient across the beam profile responsible for thermal lensing and birefringence. Due to the cost and complexity of multiple optical elements and coolant systems required, such designs have not been employed beyond research lab environments to date and do not appear destined for practical industrial usage.
The thermal gradient across the beam radius together with a positive dn/dT results in a positive thermal lens in currently used Faraday rotator optical materials. For this reason, some researchers have explored the use of negative dn/dT optical materials (such as Schott FK51 optical glass or DKDP crystals) to compensate thermal lensing in Faraday rotators [E. Khazanov et. al., “Compensation of Thermally Induced Modal Distortions in Faraday Isolators”, IEEE J. Quantum Electron. 40, 1500-1510 (2004)]. However this approach has been found to have some major drawbacks. First, these materials have different thermal conductivity, heat capacity and absorption coefficients, rendering thermal lensing compensation difficult to achieve with dynamic changes in laser power. Second, negative dn/dT materials typically have strong thermal birefringence, such that the resultant poor extinction defeats the original purpose of the optical isolator. Finally, most negative dn/dT materials studied to date have undesirable material properties such as low resistance to thermal shock and/or sensitivity to humid environments. To date, negative dn/dT materials to compensate thermal tensing have not found usage in industrial environments.
Active compensation can be considered as a viable means for reducing the effects of thermal lensing in optical isolators. However, a compact, robust, inexpensive means for accurately sensing thermal lens focal length shifts to feedback into an active thermal lens compensation system is a difficult design task. Additionally, the need for sub-Hz response times for rapid power changes while simultaneously precisely maintaining the original beam path is challenging, bulky and costly although future innovation may address these issues.
All-fiber isolators may have the potential to resolve the thermal issues noted above for high power optical isolators. Like fiber lasers, all-fiber isolators should, in principle, have beam parameters defined by the fiber waveguide characteristics. As a consequence, thermal lensing would not be expected from all-fiber isolators if the Faraday fiber and polarizing fiber which are fusion spliced together in such devices can be made to handle high power. Recently, all-fiber optical isolators of small size using short terbium glass based fiber have become available commercially for power levels up to 5 W only (AdValue Photonics Inc., 3708 E. Columbia Street, Suite 100, Tucson, Ariz. 85714. Model #AP-AFI-1060PM). Some work has been done to try to incorporate the small Faraday rotation present in low loss silica fiber into practical all-fiber optical isolators (Gerald T. Moore, “In-fiber optical isolator for high-power operation”, U.S. Pat. No. 7,336,858, Feb. 26, 2008). These research efforts however have been plagued by very large, heavy and expensive magnet structures that do not seem suitable for widespread commercial use. It remains to be seen if high rotation Faraday fiber can be made with low enough loss to support high power operation without damage with average power levels on the order 1 kW difficult to see in the foreseeable future. The effects of thermal birefringence in all-fiber optical isolators are difficult to assess presently, and represent an additional uncertainty at this point in time regarding the potential for high power operation of all-fiber isolators.
Although active thermal lens compensation or all-fiber optical isolators may prove viable in the future, a simple, low cost, completely passive means for minimizing focal shifts in high power optical isolators subject to rapid changes in power is desired.